1. Field of the Invention
The present disclosure relates to equalizers used for reproduction of digital data, and more particularly, to an apparatus and method for initializing an adaptive equalizer used in a partial response maximum likelihood filter.
2. Description of the Related Art
Generally, in hard disk drives (HDDs) and optical disk drives (ODDs), during an operation of reading and reproducing stored data, high-rate data transmission from a physical storage medium through a limited bandwidth is performed, and thus distorted data containing inter-symbol interference (ISI) is obtained. In order to recover originally stored digital data from the distorted data, a level error is corrected through a partial response (PR) method to shape digital data on which a data operation can be performed. The shaped digital data is decoded into the originally-stored digital data by a Viterbi decoding scheme using a maximum likelihood (ML) method, and is thus error-corrected on a bit basis.
In these storage mediums recovering the stored data by a partial response maximum likelihood (PRML) method, a level error is minimized using a finite impulse response (FIR) digital filter. A Viterbi decoder performs an ML bit error correction operation using the minimum path. For the optimal error correction operation, an FIR filter needs to perform a waveform equalization operation for correcting the distorted data to a level that is required for data processing in the Viterbi decoder. In an adaptive equalizer used for the above purpose, a tap coefficient of the equalizer is automatically adapted and updated using an output signal of the Viterbi decoder. In this adaptive tap coefficient updating method, consideration of a delay in the Viterbi decoder and optimization of an initial convergence coefficient in the equalizer must be properly performed.
In the case of a PRML system having a relatively small fluctuation of a channel characteristic, a separate test mode for setting an initial value of the equalizer is provided to make it possible to use the optimal one of the preset initial values. However, this initial value setting method may not show an optimal bit error rate (BER) performance during an initial convergence operation of the equalizer and in a Viterbi decoder using the initial convergence operation. Moreover, in an actual operation environment, the initial value setting method may cause system instability due to the divergence of the equalizer that results from an improper initial value caused by a change in temperature and electromagnetic environments.
FIG. 1 is a block diagram illustrating a method for initializing a conventional adaptive equalizer. Referring to FIG. 1, the conventional adaptive equalizer includes an FIR filter 100, a tap coefficient updater 110, a Viterbi decoder 120, and a BER detector 130. Analog data read out from a storage medium, such as a disk, is converted by an analog-to-digital converter (ADC) into an analog front end (AFE) output data stream {xk}. The FIR filter 100 converts the AFE output data stream {xk} into digital data of a processible level. The tap coefficient updater 110 determines optimal tap coefficients of the FIR filter 100 in response to an external control signal, and provides tap coefficient weights for the optimal tap coefficients to the FIR filter 100. The Viterbi decoder 120 corrects a bit error of a digital data stream {yk} outputted from the FIR filter 100. The BER detector 130 detects a BER of the error-corrected data stream {zk} outputted from the Viterbi decoder 120.
The analog data stored on the storage medium is physically detected at a high speed. The detected analog data is transmitted through a narrow-bandwidth channel to the ADC at a high speed, and is converted by the ADC into the AFE output data stream {xk} that is near to digital data. Unfortunately, the AFE output data stream {xk} still contains an ISI and is thus improper to be processed into digital data.
The FIR filter 100 is a kind of transversal digital filter having an FIR property, and is used to minimize a noise effect and an ISI of an input signal.
The FIR filter 100 guarantees stability because it has no feedback loop. Also, the FIR filter satisfies a linear phase property. Accordingly, the FIR filter 100 is widely used in applications that attach importance to waveform data such as the PRML of data transmission or a storage medium. The filtering property of the FIR filter 100 is determined by tap coefficients that are weights of delay taps thereof. Properly-determined tap coefficients make it possible to shape the incomplete digital data into a complete digital data, thereby minimizing the level error.
The tap coefficient updater 110 has an adaptive property that the tap coefficient is converged on a value enabling optimal performance according to the environment-dependent property of the FIR filter 100. That is, the tap coefficient updater 110 selects a tap coefficient set enabling the optimal data reproduction and provides it to the FIR filter 100.
The Viterbi decoder 120 corrects the bit error of the digital data stream {yk} outputted from the FIR filter 100. The Viterbi decoding is one of the schemes for realizing the maximum-likelihood (ML) decoding of convolution codes to provide the optimal performance for the convolutional codes.
The BER detector 130 calculates a BER of the error-corrected data stream {zk} that is induced on a read path. That is, the BER detector 130 calculates a rate of erroneous bits to the total bits of read data, and provides the calculated rate as a control signal for maintaining a proper system state.
In the conventional equalizer initializing method, during the manufacturing process for the product, the tap coefficient set of the equalizer is stored in a register and a tap coefficient providing the optimal BER performance is selected and fixed at an initial tap coefficient. However, this method needs a separate process for initializing the equalizer and a time required for finding the optimal tap coefficient. Also, there is a strong possibility that the so-determined tap coefficient may reflect only an initialized environment, failing to reflect an environment where the product is to be actually used. Moreover, the predetermined tap coefficient may be an initial value that is based on an incorrect BER detection caused by noise and/or changes in temperature and electromagnetic environments during the corresponding test process.